1. Field of the Invention
The present invention relates to a process for producing nano-particles, capable of efficiently producing monodisperse nano-particles substantially uniform in particle size and composition and freely controlling the particle size and composition, and also to nano-particles.
2. Description of the Related Art
Nano-particles, which may be of inorganic or organic compound or the like, mean particles of 500 nm or less in size, and are expected to go into various areas, e.g., cosmetics, perfumes/deodorizers, seasonings, ink jet materials, recording materials and catalysts. For example, they are expected to be applicable to highly active catalysts which can achieve a specific object in a small quantity, with their large specific area. They are also expected to exhibit functions different from those provided by the bulk materials, e.g., quantum effect, by utilizing their long light wavelengths, or their size smaller than mean free paths of electric conductors or than magnet domains.
For nano-particles to exhibit their functions, it is essential to control their size and composition. In particular, it is essential to sharply control their size and composition for them to exhibit the quantum effect. Moreover, processes for stably producing nano-particles are needed for their commercialization. However, there are many problems to be solved before they are commercialized, e.g., those involved in stabilization, dispersion, film-making, bulk material production and handling. Still more, processes for stably producing nano-particles with well-controlled morphology and interfaces are needed.
Processes for producing the nano-particles include evaporation/flocculation, vapor-phase processes, including vapor-phase reaction processes, chemical precipitation and liquid-phase processes, including solvent evaporation. Refer to Masaaki Koyama, New Ceramics, vol. 8, p. 79, 1990, Nobuyuki Kikugawa, Ceramics, vol. 34, p. 110, 1999, Hisao Suzuki, Ceramics, vol. 34, p. 76, 1999, and Advanced Technology for Nano-materials, edited by Mitsue Koizumi et al, CMC, for details.
However, none of the above nano-particle production processes can sufficiently control sharp particle size and composition. Moreover, it is difficult for these processes to freely control particle composition. For example, even for the nano-particles expected to exhibit a catalytic function, which depends only on the surface composition, the whole particle and surface are required to have the same composition.
On the other hand, a tree-shape, branched polymer has a completely uniform metal coordination number between the molecules. Therefore, it is expected that a sharp particle size and composition can be controlled, if a metal coordinated in each tree-shaped, branched polymer molecule is solidified to prepare a nano-particle. Nano-size metallic clusters which include a tree-shape, branched polymer having metal coordination capacity inside are disclosed by, e.g., Japanese Patent Application Laid-open (JP-A) No. 2001-508484, D. A. Tomalia et al, J. Am. Chem., Soc., vol. 120, p. 7355, 1998, and R. M. Crooks et al, ACC. Chem. Res., vol. 34, p. 181, 2001.
However, the process for preparing particles which include a tree-shape, branched polymer involves a practically serious problem, because the particle cannot be prepared in a controlled manner, when number of the metallic atoms it contains exceeds metal coordination number. In other words, it is difficult to obtain the particle having a size of 5.0 nm or more and containing 5000 or more metal atoms, knowing that a tree-shape, branched polymer has a metal coordination number of substantially below 5000.
Therefore, it is necessary to increase generation number of a tree-shape, branched polymer, in order to increase metal coordination number of the polymer and thereby increase size of the particle. Increasing its generation number, however, needs more reaction steps, which should greatly complicate the synthesis process. Moreover, increasing its generation number increases density of the atoms on the polymer surface, and is limited from the molecular structure. Still more, it is difficult to improve stability and functionality of the particle by forming therein a core-shell structure exhibiting a function of securing a composition on the particle surface different from that inside.
Objects and Advantages
It is an object of the present invention to provide a process for producing nano-particles, capable of efficiently producing monodisperse nano-particles substantially uniform in particle size and composition. It is another object of the present invention to provide nano-particles produced by the same process.